Many scientists believe that the universe began with an enormous explosion called the “Big Bang”, happened about 13.7 billion years ago.
A. that
B. which
C. when
D. what
网友回答
A. that
(答案→)B. which
C. when
D. what
解析：考查定语从句。本题考查的是非限制性定语从句，which指代前面一句话的内容，在从句中做主语。句意：很多科学家都认为宇宙起源于宇宙大爆炸，这是发生在137亿年前的事情了。That不能引导非限制性定语从句，when在句中不能做主语；what不能引导定语从句。故B正确。